Question

I’ve always been interested in how you can throw some SQL at database, and it nearly instantaneously returns your results in an orderly manner without thinking about it as anything other than a black box.

What is really going on?

I’m pretty sure it has something to do with how values are laid out regularly in memory, similar to an array; but aside from that, I don’t know much else.

How is SQL parsed in a manner to facilitate all of this?

Answer 1

The engine builds a such called query plan.

It’s a set of algorithms used to return the sets that you described logically with an SQL query.

Almost each engine lets you see what query plan will it build for a certain query.

• In MySQL and PostgreSQL, you prepend your query with the word EXPLAIN

• In SQL Server, you run SET SHOWPLAN_TEXT ON before running the query or just press Ctrl-L in the Management Studio

• In Oracle, you prepend the query with EXPLAIN PLAN FOR and then issue SELECT * FROM (dbms_xplan.display)

You may find interesting this article in my blog:

• Double-thinking in SQL

which addresses the same question.